Abnormal Labor

ABNORMAL LABOR

Abnormal labor, or labor dystocia (literally, “difficultlabor or childbirth”) is characterized by the abnormal pro-gression of labor. Dystocia is the leading indication for primary cesarean delivery in the United States. Despite thehigh prevalence of labor disorders, considerable variability exists in the diagnosis, management, and criteria for dystocia that re-quires intervention. Because dystocia can rarely be diag-nosed with certainty, the relatively imprecise term “failure to progress” has been used, which includes lack of pro-gressive cervical dilation or lack of descent of the fetal head or both.

Factors That Contribute to Normal Labor— The Three Ps

Labor is the occurrence of uterine contractions of suffi-cient intensity, frequency, and duration to bring about demonstrable effacement and dilation of the cervix. Dystocia results from what have been categorized classi-cally as abnormalities of the “power” (uterine contractions or maternal expulsive forces), “passenger” (position, size, or presentation of the fetus), or “passage” (pelvis or soft tissues).

UTERINE CONTRACTIONS (“POWER”)

Uterine activity can be monitored by palpation, external tocody-namometry, or by using intrauterine pressure catheters (IUPCs) (Fig. 9.1). A tocodynamometer is an external strain gauge that is placed on the maternal abdomen. It records the frequency of uterine contractions and relaxations, as well as the duration of each contraction. An IUPC, in ad-dition to recording contraction frequency and duration, also directly measures the pressure generated by uterine contractions, via a catheter inserted into the uterine cav-ity. The catheter is attached to a gauge that measures intra-uterine pressure in millimeters of mercury (mm Hg).

Recent studies suggest that the use of an IUPC instead of external tocodynamometry does not affect the outcome in cases of abnormal labor.

However, an IUPC may be useful in specific situations, such as maternal obesity or other factors that may prevent accurate clinical evaluation of uterine contractions.

For cervical dilation and fetal descent to occur, each uterine contraction must generate at least 25 mm Hg of peak pressure. Optimal intrauterine pressure is 50 to 60 mm Hg. The frequency of uterine contractions is also impor-tant in generating a normal labor pattern: the optimal fre-quency of uterine contractions is a minimum of three contractions in a 10-minute interval, often described as “adequate.” Uterine contractions that are too frequent are not optimal, because they prevent intervals of uterine re-laxation. During this “rest interval,” the fetus receives unimpeded uteroplacental blood flow for oxygen and waste transport. Without these rest periods, fetal oxygenation may be compromised.

Another unit of measure commonly used to assess con-tractile strength is the Montevideo unit (MVU). This unit is the number of uterine contractions in 10 minutes times the average intensity (above the resting baseline intrauterine pressure). Normal progress of labor is usually associated with 200or more Montevideo units.

FETAL FACTORS (“PASSENGER”)

Evaluation of the passenger includes clinical estimation of fetal weight and clinical evaluation of fetal lie, presenta-tion, position, and attitude. If a fetus has an estimated weightgreater than 4000 to 4500 grams, the risk of dystocia, includ-ing shoulder dystocia and fetopelvic disproportion, is greater. Because ultrasound estimation of fetal weight is often in-accurate by as much as 500 to 1000 grams when the fetus is near term (40 weeks’ gestational age), this information must be used in conjunction with other parameters when making management decisions.

Fetal attitude, presentation, and lie also play a role in the progress of labor (Fig. 9.2). If the fetal head is asynclitic (turnedto one side; asynclitism) or extended (extension), a larger cephalic diameter is presented to the pelvis, thereby increas-ing the possibility of dystocia. A brow presentation (about 1 in 3000 deliveries) typically converts to either a vertex or face presentation, but, if persistent, may cause dystocia re-quiring cesarean delivery. Likewise, a face presentation (about 1 in 600 to 1000 deliveries) requires cesarean deliv-ery in most cases. However, a mentum anterior presenta-tion (chin toward mother’s abdomen) may be deliveredvaginally if the fetal head undergoes flexion, rather than the normal extension. A persistentoccipitoposterior positionis alsoassociated with longer labors (approximately 1 hour in multiparous patients and 2 hours in nulliparous patients). Incompoundpresentations, when one or more limbs prolapse alongsidethe presenting part (about 1 in 700 deliveries), the extrem-ity usually retracts (either spontaneously or with manual as-sistance) as labor continues. When it does not, or in the 15% to 20% of compound presentations associated with umbilical cord prolapse, cesarean delivery is required.

Fetal anomalies, such as hydrocephaly and soft tissue tumors, may also cause dystocia. The routine use of prena-tal ultrasound for other causes has allowed identification of these situations, significantly reducing the incidence of un-expected dystocia of this kind.

MATERNAL FACTORS (“PASSAGE”)

A number of maternal factors are associated with dystocia. Dystocia can result from maternal skeletal or soft-tissue anomalies that obstruct the birth canal. Cephalopelvicdisproportion, in which the size of the maternal pelvis isinadequate to the size of the presenting part of the fetus, may impede fetal descent into the birth canal.

Clinical, radiographic, and CT measurements of the bony pelvis are poor predictors of successful vaginal delivery, due to the inaccuracy of these measurements as well as case-by-case differences in fetal accommodation and mechanisms of labor.

Clinical pelvimetry, the manual evaluation of the diam-eters of the pelvis, is also a poor predictor of successful vaginal birth, except in rare circumstances when the pelvic diameters are so small as to render the pelvis “completely contracted.” Although radiographic and CT pelvimetry can be helpful in some cases, the progress of descent of the presenting part in labor is the best test of pelvic adequacy.

Soft-tissue causes of dystocia include abnormalities of the cervix, tumors or other lesions of the colon or adnexa, distended bladder, uterine fibroids, an accessory uterine horn, and morbid obesity. Epidural anesthesia may con-tribute to dystocia by decreasing the tone of the pelvic floor musculature.

Risks

Dystocia may be associated with serious complications for both the woman and the fetus. Infection (chorioamnionitis) is aconsequence of prolonged labor, especially in the setting of ruptured membranes. Fetal infection and bacteremia, including pneumonia caused by aspiration of infected amniotic fluid, is linked to prolonged labor. In addition, there are the attendant risks of cesarean or operative de-livery, such as maternal soft tissue injury to the lower genital tract and fetal trauma

Diagnosis and Management of Abnormal Labor Patterns

Graphic documentation of progressive cervical dilation and effacement facilitates assessing a patient’s progress in labor and identifying abnormal labor patterns. The Friedman Curve is commonly used for this purpose. Labor abnormalities can be categorized into two general types: protraction disorders, in which labor is slow to progress, and arrest disorders, in which labor ceases to progress (Table 9.1). Protraction can occur dur-ing both the latent and active phases of labor, while arrest is recognized only in the active phase. Although the defi-nition of the latent phase of labor is controversial, in gen-eral it can be defined as the phase in which the cervix effaces but undergoes minimal dilation.

Management of abnormal labor encompasses a wide range of options, from observation to operative or cesarean delivery. Management choice depends on several factors:

·      Adequacy of uterine contractions

·      Fetal malposition or cephalopelvic disproportion

·              Other clinical conditions, such as nonreassuring fetal status or chorioamnionitis

Management decisions should be balanced between ensur-ing a positive outcome for mother and fetus and avoiding the concomitant risks of operative and cesarean delivery. 

FIRST-STAGE DISORDERS

prolonged latent phase is one that exceeds 20 hours in anulliparous patient or 14 hours in a multiparous patient. A pro-longed latent phase does not necessarily predict an abnormal ac-tive phase of labor. Some patients who have initially beendiagnosed as having a prolonged latent phase are subse-quently found to have been in false labor. A prolonged la-tent phase does not in itself pose a danger to the mother or fetus. Options for management of women with a pro-longed latent phase of labor include observation and seda-tion. With either of these options, the patient may stop having contractions, in which case she is not in labor; may go into active labor; or may continue experiencing pro-longed labor into the active phase. In the latter case, other interventions as described below may be administered to augment uterine contractions.

Once the patient is in active labor, the first stage is consid-ered prolonged when the cervix dilates less than 1 cm per hour in nulliparous women, and less than 1.2 to 1.5 cm per hour in multi-parous women. Management options for a prolonged firststage include observation, augmentation by amniotomy or oxytocin, and continuous support. Cesarean delivery usu-ally is warranted if maternal or fetal status becomes non-reassuring.

Augmentation Augmentationrefers to stimulation ofuterine contractions when spontaneous contractions have failed to result in progressive cervical dilation or de-scent of the fetus. Augmentation can be achieved with amniotomy (artificial rupture of membranes) or oxy-tocin administration. Augmentation should be consideredif the frequency of contractions is less than 3 contractions per 10 minutes or the intensity of contractions is less than 25 mm Hg above the baseline or both. Before augmentation, the mater-nal pelvis and cervix as well as fetal position, station, and well-being should be assessed. If there is no evidence of disproportion, oxytocin can be used if uterine contractions are judged to be inadequate. Contraindications to aug-mentation are similar to those for labor induction .

If the membranes have not ruptured, amniotomy may en-hance progress in the active phase and negate the need for oxytocin augmentation. Amniotomy allows the fetal head, rather thanthe otherwise intact amniotic sac, to be the dilating force. It may also stimulate the release of prostaglandins, which could aid in augmenting the force of contractions.

Amniotomy is usually performed with a thin, plastic rod with a sharp hook on the end. The end is guided to the open cervical os with the examiner’s fingers, and the hook is used to snag and disrupt the amniotic sac. Risks of amniotomy include fetal heart rate decelerations due to cord compression and an increased incidence of chorioamnionitis. For these reasons, amniotomy should not be routine and should be used for women with pro-longed labor. The fetal heart rate (FHR) should be eval-uated both before and immediately after rupture of the membranes. 

It has been shown that amniotomy combined with oxytocin administration early in the active stage reduces labor by up to 2 hours, although there is no change in the rate of cesarean delivery with this treatment protocol.

The goal of oxytocin administration is to effect uterine ac-tivity sufficient to produce cervical change and fetal de-scent while avoiding uterine hyperstimulation and fetal compromise. Typically, a goal of a maximum of 5 contrac-tions in a 10-minute period with resultant cervical dilation is considered adequate. Oxytocin may be administered in low-dose or high-dose regimens. Low-dose regimens are associated with a decreased incidence and severity of uterine hyperstimulation. High-dose regimens are associated with decreased labor times, incidence of chorioamnionitis, and cesarean delivery for dystocia.

Continuous Labor Support Continuous support duringlabor from caregivers (nurses, midwives, or lay individuals) may have a number of benefits for women and their new-borns. Continuous care has been associated with reduced need for pain relief and oxytocin administration, lower rates of cesarean and operative deliveries, decreased incidence of 5-minute Apgar scores lower than 7, and increased patient satisfaction with the labor experience. However, there are insufficient data comparing differences in benefits on the basis of level of training of support personnel—that is, whether the caregivers were nurses, midwives, or doulas. There is no evidence of harmful effects from continuous support during labor.

SECOND-STAGE DISORDERS

A second-stage protraction disorder should be considered when the second stage exceeds 3 hours if regional anesthesia has been ad-ministered, or 2 hours if no regional anesthesia is used, or if the fetus descends at a rate of less than 1 cm per hour if no regional anesthesia is used. Second-stage arrest is diagnosed when there is no descent after 1 hour of pushing. In the past, the fetus wasthought to be at increased risk for morbidity and mortality when the second stage exceeded 2 hours. Currently, more intensive intrapartum surveillance provides the ability to identify the fetus that may not be tolerating labor well.

Thus, the length of the second stage of labor is not in itself an absolute or even a strong indication for operative or cesarean delivery.

As long as heart tones continue to be reassuring and cephalopelvic disproportion has been ruled out, it is con-sidered safe to allow the second stage to continue. If uterine contractions are inadequate, oxytocin adminis-tration can be initiated or the dosage increased if already in place.

Bearing down efforts by the patient in conjunction with uterine contractions help bring about delivery. Labor positions other than the dorsal lithotomy position (e.g., knee-chest, sitting, squatting, or birth-chair) may bring about subtle changes in fetal presentation and facilitate vaginal delivery. Fetal accommodation may also be facili-tated by allowing the effects of epidural analgesia to dissi-pate. The absence of epidural analgesia may increase the tone of the pelvic floor muscles, facilitating the cardinal movements of labor and restoring the urge to push. In some cases of fetal malpresentation, manual techniques can facilitate delivery. If the fetus is in the occipitoposte-rior position and does not spontaneously convert to the nor-mal position, rotation can be performed to turn the fetus to the anterior position (Fig. 9.3).

The decision to perform an operative delivery in the second stage versus continued observation should be made on the basis of clinical assessment of the woman and the fetus and the skill and training of the obstetrician. Nonreassuring status of the fetus or mother is an indica-tion for operative or cesarean delivery.